I think you're mixing several things. Commands are executed from the command-line interface. Doing work on a JSON request would be something completely different that you should handle with a controller. It would actually be no different from doing it in
MVC without Orchard. Look up "AJAX ASP.NET MVC" for some pointers.

The idea behind commands is that many tasks can be automated this way, which will be a real time saver to people building lots of sites with Orchard. Ideally, features would be exposed both as admin UI and commands, as relevant. They are not really an extension
to MVC, more like an orthogonal concern.